The present invention relates to a roots pump having a plurality of rotary shafts arranged in parallel relation to each other and a rotor arranged on each of the rotary shafts, wherein the rotors on any two adjacent rotary shafts are engaged with each other and a set of engaged rotors are accommodated in a pump chamber which is formed in the roots pump.
In a roots pump, or a kind of vacuum pump, as disclosed in Unexamined Japanese Patent Publication No. 2002-221178, a set of two rotors are rotated in engagement with each other. The rotation of the two engaged rotors transfers gas while simultaneously compressing the same. The roots pump having plural sets of such two rotors has a passage formed in an end wall which partitions any two adjacent pump chambers which are arranged in the axial direction of the rotary shaft for transferring the gas from a large-volume pump chamber to a smaller-volume pump chamber.
The roots pump has a closed space formed between the rotor and a peripheral wall surface of the pump chamber facing the rotor, and such closed space communicates with the other closed space formed between the other rotor and the other peripheral wall surface of the same pump chamber facing the other rotor. The passage in the end wall communicates with this confluent space (a confluent passage in the present invention), and gas in the confluent space flows into the passage in the end wall.
When a roots pump is used as a vacuum pump in a film production process for manufacturing semiconductor, such as process for forming nitriding film, reaction product flows into the roots pump with gas. When this reaction product is accumulated in a pump chamber of the roots pump, operation of the roots pump may become disabled. Therefore, there has been a need for preventing reaction product from being accumulated in a roots pump.